Amplifiers are used in a wide variety of applications. In general, amplifiers are used to increase the power of signals. For example, amplifiers can be used to convert low-power radio frequency (RF) signals into larger RF signals for driving the antenna of a transmitter. In such cases, amplifiers may be implemented as part of an overall RF transmission system.
Power amplifiers tend to consume a significant portion of the total power consumed by a transmission system. Therefore, the power amplifier's efficiency (i.e., the power of the amplifier output signal divided by the total power consumed by the amplifier) is an amplifier quality that designers consistently strive to increase.
One method used to distinguish the type of amplifier is by “class”. The amplifier class represents the amount of time the transistor is conducting current over one cycle when excited by a sinusoidal input signal. In general, classes such as A, B and C are conduction angle amplifiers defined by the length of their conductive state over one cycle when excited by a sinusoidal input signal. In these amplifiers the output stage is being driven to an intermediate level between being fully on and fully off. In contrast, classes such as D and E, are “switching” amplifiers that that are constantly switched between fully on and fully off. Other amplifiers, such as class F (and inverse class F) are conduction angle amplifiers where additional methods are employed to shape the transistor current and voltage waveforms to yield improvements in parameters such as amplifier output power and efficiency.
Specifically, class F and inverse class F amplifiers are amplifiers that can boost efficiency and output power by shaping the output waveform into a near square wave with the use of harmonic resonators. So implemented, class F and inverse class F amplifiers can provide efficiencies of more than 90 percent in some applications. Such high efficiency amplifiers are increasingly desirable in radio frequency (RF) mobile communication applications. For example, high efficiency is increasingly important in mobile phones, tablets, and other devices where long battery life is highly desirable. Unfortunately, implementing high efficiency class F amplifiers can be problematic. For example, the device output capacitance in many devices can prevent the effective and efficient implementation of high efficiency RF amplifiers for high power applications.